New Fossils from the Caldecott Tunnel

"Paleo mitigation" work during the dig of the new Caldecott Tunnel bore has yielded fresh insight into a familiar set of rocks thanks to fossils like this Miocene horse molar. All photos courtesy CalTrans and PaleoResource Consultants.

The fourth bore for the Caldecott Tunnel, which connects Oakland and points west with Orinda and points east, is being dug day and night with the first drive-throughs slated for 2013. Part of the job is what Caltrans contracts call "paleontological mitigation": allowing fossil scientists to salvage resources—"fossils and the deposits they are found in"—that would otherwise be destroyed. A paleo mitigation agreement governs the Caldecott fourth bore project, and scientists shared some of the early results earlier this month.

Fossil hunting is tedious, exacting work, but there are those who love it. Kristin McCallister, a paleontologist for contractor PaleoResource, is shown here during early work at the job site, examining spoils.

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The vast majority of fossils are microscopic and consist of plankton or pollen grains. But the big bones are sexy. Let's look at the pictures PaleoResource's Lanny Fisk showed at the CalPaleo 2011 meeting in Rocklin, starting with the leg bone of a rhinoceros.

Here are two vertebrae of a Miocene camel species.

And this is the metatarsal bone of an ancient horse.

Finally here's the upper jaw of an oreodont, an extinct browsing mammal the size of a sheep.

All of these (plus bird, pronghorn and wolverine bones) were salvaged from rocks of the Orinda Formation that are about 11 million years old, a time late in the Miocene Epoch. The researchers also found hundreds of plant fossils for the first time in these rocks, including this laurel leaf.

The Orinda Formation makes up the eastern end of the bore. In the middle is the Claremont Formation, a cherty unit that holds up the highest part of the Berkeley Hills. This 1-inch clam is from the Claremont.

Obviously this shows a change from a land to a sea environment as we go back in time.

At the west end of the bore is the still older Sobrante Formation. Fisk told the CalPaleo audience that this unit has yielded a large fish fossil fauna as well as some leaves and marine invertebrates (clams, crabs and plankton). He was particularly excited about the fish, which somewhat resemble cool-water fish of this age from northwestern Oregon.

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One handicap that Fisk mentioned was difficulty getting into the tunnel, where paleontologists can watch the rock face and stop operations long enough to examine and salvage the best fossils. Working the spoils pile isn't good for getting exact locations and geological context for fossils. But the bore will be carried out in two passes, and maybe we can get another look at this unique dig.